The persistent state of subclinical inflammation, which characterizes obesity, is thought to play an important role in the pathophysiological link between obesity and cardiometabolic disease risk. Central to this 'inflammatory hypothesis' is the role of adipose tissue macrophages (ATMs) in mediating the inflammatory response to chronic over-nutrition. Despite displaying reductions in adipose tissue (AT) inflammation and insulin resistance (IR) following high fat feeding, Toll-like Receptor 4 (TLR4) deficient mice exhibit a marked accumulation of ATMs. Preliminary data from our laboratory are consistent with the idea that TLR4 deficiency reduces AT inflammation and IR in diet-induced obesity by shifting ATM polarization towards an, anti- inflammatory, alternatively activated state. To date, no studies have explored the role of TLR4 signaling in ATM polarization. Thus, the Specific Aims of this project are: 1) to determine the influence of hematopoietic cell TLR4 signaling on AT inflammation and IR in diet-induced obesity; 2) to determine the influence of parenchymal and hematopoietic cell TLR4 signaling on the in vivo polarization of ATMs in diet-induced obesity; 3) to determine the influence of TLR4 signaling on macrophage substrate metabolism and lipid accumulation. To accomplish these aims we will utilize a bone marrow transplant (BMT) model, whereby C57BL/6 (WT) and TLR4-/- mice will receive either WT or TLR4-/- BM following lethal irradiation. Subsequently, recipient mice will be placed on a high fat diet (HFD) for 16 weeks. AT inflammation and IR will be assessed via AT expression of inflammatory markers and hyperinsulinemic-euglycemic clamps, respectively. A combination of real time rtPCR, flow cytometry, and immunofluorescent staining, and mixed chimeras will be used to determine the influence of parenchymal and hematopoietic cell TLR4 signaling on ATM polarization. Finally, we will utilize radio-labeled substrates to determine the influence of TLR4 signaling on macrophage substrate metabolism and neutral lipid synthesis at baseline and following Th1 and Th2 cytokine stimulation. Our hypotheses are: 1) hematopoietic cell TLR4 deficiency will reduce AT inflammation and IR following a HFD. Conversely, the restoration of hematopoietic cell TLR4 signaling in TLR4-/- mice will exacerbate AT inflammation and IR following a HFD; 2) Hematopoietic cell TLR4 signaling will be the primary determinant of ATM polarization. That is, TLR4-/- ATMs will be primarily M2 polarized WT ATMs will primarily display an M1 phenotype regardless of the parenchymal cell genotype; 3) TLR4 deficiency will reduce ATM lipid accumulation and promote a shift in substrate metabolism towards fatty acid oxidation.